Because of their excellent rigidity, heat resistance and impact resistance, propylene polymers are used for various molded articles in many fields.
For further improving the impact resistance of propylene polymers, there is known a process comprising initially homopolymerizing propylene and then copolymerizing propylene and ethylene to prepare a block copolymer.
For example, Japanese Patent Laid-Open Publication No. 337308/1992 discloses a process for preparing a block copolymer in which propylene is first homopolymerized or copolymerized with ethylene in the presence of a transition metal compound having as a ligand a cyclopentadienyl bridged with a silylene group and an organoaluminum compound so as to produce a propylene homopolymer or a propylene copolymer containing less than 6% by weight of ethylene, in an amount of 40 to 95% by weight based on the total weight of the final polymer product; and then ethylene and propylene in a weight ratio of 10/10 to 95/5 are further copolymerized in the presence of the same transition metal compound and the same organoaluminum compound until a copolymer is produced in an amount of 60 to 5% by weight based on the total weight of the final polymer product. This publication also describes that the block copolymer obtained by this process has well balanced properties between impact resistance and rigidity.
Japanese Patent Laid-Open Publication No. 202152/1993 discloses a process for preparing a polypropylene molding material comprising (1) 20 to 99% by weight of a crystalline polymer having a propylene unit content of not less than 95% by weight and (2) 1 to 80% by weight of a non-crystalline ethylene-propylene copolymer having an ethylene unit content of 20 to 90% by weight, in the presence of a catalyst comprising a transition metal compound and an organoaluminum compound, in which polymerization to prepare the non-crystalline ethylene-propylene copolymer is carried out using a specific bridged metallocene compound and aluminoxane. This publication also describes that the polypropylene molding material obtained by this process has excellent properties, particularly in low-temperature impact strength.
However, there has recently been an increasingly severe demand for properties of polypropylene compositions. In addition, there has also been a need for a propylene polymer composition having well balanced properties among rigidity, heat resistance and impact resistance, and a process capable of preparing such composition.